With the emerging market for the wireless LAN (local area network) standards, the need exists to provide radio solutions that integrate these standards together with the popular 802.11b standard into a single receiver. Preferably, a single VCO in combination with selectable frequency division capable of covering the 802.11a, b and g frequency bands, i.e., 2.4–2.5, 2.4–2.6 and 5.1–5.8 GHz, respectively, is needed. However, the obtainment of a wide VCO tuning range in combination with a low tuning constant Kvco is ever more challenging. A low Kvco, is desirable for the PLL (phase-locked-loop) design and for the minimization of the VCO's sensitivity to noise and supply variations.
Band switching in addition to differential VCO tuning are known methods used to reduce the Kvco over the extended frequency range. Differential VCO tuning also provides significant reduction in up-converted common-mode (bias) noise into phase noise and in the oscillator's sensitivity to supply- and bias variations. Several techniques exist to implement differential tuning. As the varactor's capacitance is determined by the voltage across its terminals, one can decouple a varactor capacitively from the oscillator's output nodes and bias both its terminals differentially. However, this will reduce the oscillation swing across the varactor, resulting in a highly non-linear tuning curve.
Alternatively, p-type and n-type varactors can be combined using simple NMOS and PMOS transistors in inversion mode. However, standard MOSFET transistors, used in differentially tuned VCO are not optimized for a maximum Cmax/Cmin-ratio or Q. Also, the C(V) curves of NMOS and PMOS devices are not well matched and can cause a loss of CMRR (Common-Mode Rejection Ratio), i.e. the circuit's ability to reject variations in its common-mode tuning levels that affect the frequency of oscillation.
Another approach is to use a combination of p- and n-type accumulation-depletion mode varactors. However, this requires a triple well process that adds to the cost. Finally, one could use only PMOS accumulation-depletion varactors, and connect the gates of one set of varactors to the outputs and tune it through the well side, and connect the well sides of a second set of varactors to the outputs and tune this set through the gate sides. However, in that case the oscillator is loaded with the large, low-Q parasitic capacitance between the well and the substrate; this will negatively affect the oscillator's phase noise and tuning range.
Hence, there is a need for a VCO differential tuning device, i.e, a tuner, that allows a VCO to be tuned differentially and that preserves maximum oscillation swing across the varactors and thus maximizes the tuning linearity of the VCO.